The latest data sent back from the Cassini spacecraft, along with previously-gathered results, have confirmed that the mysterious moon, Phoebe, is a remnant from the early days of our solar system.

Astronomers suspected Phoebe was a captured moon, and now they have the proof: composition data sent back by Cassini shows that of all the objects we have studied in the solar system, Phoebe is most similar to Pluto. She matches the composition expected of the objects in the Kuiper Belt, an icy region beyond Neptune, and thought to be the place most comets come from. Indeed, some speculate that Pluto itself is a Kuiper Belt object, not a planet at all.

Torrence Johnson, on the Cassini science team at the Jet Propulsion Laboratory, said: "This is pretty good evidence that Phoebe was put together in the outer parts of the solar system."

So, at more than four billion years old, Phoebe is a battered, but otherwise perfectly-preserved example of the protoplanetary building blocks that eventually combined to form the planets.

Speaking to Nature, Johnson explains that the outer solar system would have been chock-full of icy planetesimals like Phoebe. Many of these bodies collected together to form the cores of the gas giants. Most of the others were swept into the outer solar system by the gravitational effects of these new, huge, planets. Phoebe remained behind, dragged into her orbit of Saturn as the planet was swallowing the streams of gas that make up its atmosphere.

Despite having taken a heavy battering over the last four billion years, Phoebe is still mostly round with an average diameter of 214 kilometres. The spherical shape is broken up by one especially large crater whose walls are 16 kilometres high.

The details of Phoebe's radius, combined with its orbit, reveal the body's composition. It is approximately 50 per cent rock - very like Pluto - and strikingly different from Saturn's other moons, which are around 35 per cent rock.

As for the rest of the moon, the data shows frozen carbon dioxide, water ice, hydrocarbons and iron minerals all present on the surface. As well as proving that Phoebe is no asteroid, this shows that chemically at least, Phoebe is very similar to the comets, also Kuiper belt objects. ®

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